This invention relates generally to battery charging methods, and more particularly to charging battery cells while dynamically compensating for parasitic cable impedance.
Rechargeable battery cells are typically charged with chargers and power supplies that connect to a host device through a copper cable. For example, in a cellular telephone application, a power supply might plug into a wall receptacle, with a three to six foot cable running from the supply to the phone. These cables, like any metal conductor, have a parasitic impedance. They are not perfect conductors in that when current passes through the cable, power is dissipated due to the characteristic resistivity (xcfx81) of the conductor. This dissipated power is lost as heat, which reduces the efficiency of the charging system.
The parasitic power loss is problematic to charger designers. Lithium batteries have very sensitive voltage thresholds that cannot be exceeded. For example, the integrity of a lithium cell rated at 4.1 V can be compromised if charged to a voltage above this limit. At the same time, the voltage of a lithium battery is a direct indicator of battery capacity. Thus, the goal is to charge the battery as quickly as possible to 4.1 V, without exceeding this limit.
Chargers that employ a cable to connect to the battery must measure the termination voltage of the battery at the charger side of the cable. As a result, the actual voltage measured by the charger is the battery cell voltage plus the voltage drop across the cable. As the cable impedance can be as high as 600 mxcexa9, and as the charging current can be as high as 1 amp, the actual measured voltage at the charger can be in error by as much as 600 mV! Additionally, the impedance of the cable can vary with the quality of the connection and temperature of the conductor.
One possible solution to this erroneous measurement is to take the voltage measurement on the battery side of the cable. The problem with this method is that an extra conductor in the cable is required to transmit the voltage information back to the charger. This adds cost and weight to the charging system.
Chargers generally charge initially at a very high current (rapid charge) until the cell reaches its termination voltage, and then at a very low rate (trickle charge) until the battery is fully charged. The longer that a charger remains in rapid charging mode, the quicker the battery will charge. Erroneous voltage measurements mean that the charger will begin to taper charging current before charging is complete. In other words, the charger will sense 4.1 V (and therefore terminate rapid charging) before the cell actually hits 4.1V because there will be a 600 mV parasitic voltage drop across the cable. Since the charger does not know what the impedance of the cable is, it must assume a zero impedance and thus terminate rapid charging when the charger side of the cable reaches 4.1 V. The net effect is that a battery takes longer to charge than it should.
The charger""s goal is to accurately and quickly charge the cell. Parasitic cable impedance interferes with this goal. There is thus a need for a faster charging algorithm which compensates for cable impedance.